1. Field of the Invention
This invention relates generally to methods and apparatus for reducing the emissions of diesel engines and more specifically to methods and apparatus for reducing the emissions of diesel engines intended for use in underground mines and other similar or potentially inflammable or inadequately ventilated environments.
2. Background of the Invention
Diesel engines are used to power a wide variety of vehicles and equipment used in various underground and mining applications due to their improved safety and efficiency over electrically powered vehicles and equipment. However, there are substantial problems associated with the use of diesel engines in such underground environments that have not yet been solved.
One problem is that is not unusual for the air in underground mines to contain various ignitable dusts or explosive gases, most likely methane. Unfortunately, a diesel engine is prone to emit sparks or flames caused by backfiring through the intake or exhaust manifolds, which may cause a fire or explosion if the atmosphere surrounding the engine contains certain concentrations of the ignitable dusts or explosive gases. Moreover, since the internal operating temperatures of such engines may exceed 1200.degree. F., the external surfaces of the engines may be heated to extremely high temperatures. Such high surface temperatures can trigger a fire or explosion if ignitable dusts accumulate on the hot external surfaces of the engine or if inflammable liquids come in contact with these hot surfaces.
Another factor which has heretofore limited the use of diesel engines in mines or other environments having limited ventilation, and with increasing environmental awareness, is becoming a concern in all diesel engine operations, is that the exhaust from such engines contains numerous components thought to be harmful to humans, such as unburned hydrocarbons, carbon monoxide (CO), oxides of nitrogen (NO.sub.x), sulfur dioxide (SO.sub.2), sulfates, as well as solid particulate matter. Generally speaking, the solid particulate matter in the diesel engine exhaust comprises small, solid, irregularly shaped particles, which are themselves agglomerates of smaller sub-particles. The solid particulate matter may often have high molecular weight hydrocarbons absorbed on their surfaces. Frequently, the particulate matter is a complex mixture of pure carbon and various kinds of organic materials, and the sizes may range from very small particles of about 0.01 microns to relatively large clusters in the range of 10-30 microns, giving the particulate an extremely fine and light, flour-like consistency. Turbocharged diesel engines tend to emit more of the smaller particles with much lower levels of retained organic compounds. Particle sizes of 10 microns and less are considered to be the most damaging to human lungs, and certain characteristic components of diesel exhaust particulate emissions are known carcinogens.
Particulate emission can be reduced by limiting the power output of the diesel engine, and manufacturers can reduce particulate emissions by limiting the amount of fuel injected under acceleration and high load (i.e., lug-down) conditions. However, reducing the amount of fuel injected during acceleration and lug-down operations is not effective to eliminate all solid particulate emission, or even decrease it to a desirably low level, unless the power output of the engine is reduced to an unacceptably low level.
Consequently, several alternative systems have been developed in recent years in attempts to find a more effective means of reducing the solid particulate emissions of diesel engines in hopes of making the engine exhaust cleaner, thereby enhancing the diesel engines' environmental acceptability, and making them more suitable for underground use. Principle among these alternative systems are water scrubbing systems, systems that thermally (i.e., catalytically) oxidize the particulate matter while it is still entrained in the exhaust stream, systems for thermally oxidizing filter-trapped particulate matter, and systems for catalytically oxidizing filter trapped particulate matter. Some examples of such alternative systems are disclosed in U.S. Pat. No. 3,786,635 issued to Kates et al., U.S. Pat. No. 3,886,738 issued to Sien, U.S. Pat. No. 3,903,694 issued to Aine, U.S. Pat. No. 4,075,994 issued to Mayer et al., U.S. Pat. No. 4,133,654 issued to Hill et al., U.S. Pat. No. 4,338,784 issued to Liu et al., U.S. Pat. No. 4,345,429 issued to Yasuhara, U.S. Pat. No. 4,671,060 issued to Wilkens, and U.S. Pat. No. 4,864,821 issued to Hoch.
Unfortunately, none of these systems has proven to be a panacea, and there remain a number of serious shortcomings which have tended to limit their success, particularly in the underground mining environment. For example, most water scrubbing systems, such as those disclosed by Sien and Hill et al., typically comprise a water-filled baffle chamber that is connected to the exhaust manifold of the engine. The exhaust gases from the engine are bubbled through the water in the chamber, which cools the exhaust gases and remove a small percentage (about 10%) of the solid particulate matter. Since the exhaust gases pass through a water bath most water scrubbing systems make excellent flame arresters, which, of course, has made them attractive for use in inflammable atmospheres, such as those typically associated with underground mines. Unfortunately, however, water scrubbers consume relatively large amounts of water and must be thoroughly cleaned at very frequent intervals. While water scrubbers do initially remove some of the water-soluble sulfur dioxide (SO.sub.2) from the exhaust gases, they cannot remove carbon monoxide, oxides of nitrogen, or other gaseous pollutants from the exhaust gases. However, even the removal of the sulfur dioxide creates problems because the absorbed sulfur dioxide reacts with the water to form sulfuric acid (H.sub.2 SO.sub.4). This sulfuric acid is eventually emitted from the exhaust system along with the exhaust gases, which causes irritation of the upper airways of miners working in the vicinity of the engine. Furthermore, recent changes in the laws regulating the emissions of mine certified diesel engines have tightened the emission requirements to the point were most water scrubbers just cannot meet the new, more rigorous emission requirements.
The patent issued to Wilkens uses a "dry" heat exchanger to cool the exhaust gases to avoid some of the problems associated with the water scrubbers. Unfortunately, Wilkens' heat exchanger accumulates soot deposits quite rapidly, which significantly reduces the thermal transfer efficiency of the heat exchanger. Consequently, Wilkens' heat exchanger must be disassembled and thoroughly cleaned at frequent intervals; an expensive and time-consuming process. Moreover, Wilkens' system cannot meet the new exhaust emission requirements, because it does not remove any of the solid particulate matter or gaseous pollutants from the engine exhaust.
One method for reducing the amount of particulate matter in diesel engine exhaust is instream thermal oxidation. Disadvantageously, instream thermal oxidation techniques require the provision to the exhaust stream of large amounts of heat energy to further oxidize the particulate matter, which heat is usually unrecoverable, thus reducing efficiency of the system. Catalytic instream oxidation methods, such as those disclosed by Mayer et al. and Yasuhara do not require additional energy, but they also do not solve the problem of devising a suitable means for introducing the catalyst material into the exhaust stream without raising the surface temperature of the exhaust manifold or the temperature of the exhaust gases themselves above the maximum allowable safe temperatures.
Another method of reducing the solid particulate emissions is to use a filter to trap the particulate matter before it escapes into the surrounding atmosphere. Ceramic materials, stainless steel wire mesh, and other filter materials capable of withstanding the high-temperature exhaust gases have been tried and are being used. The patents issued to Hoch and Yasuhara disclose variations on this theme. Unfortunately, because of the large quantities of particulate matter that are generated by most diesel engines, such filters clog quickly, which increases back pressure in the engine exhaust and affects the performance and efficiency of the engine. Of course, replacing the filter when the back pressure exceeds some predetermined limit would be helpful. However, the metal or ceramic materials used in most effective filters are expensive, so it is simply not practical to throw such filters away when they are clogged. Several filter regeneration methods have been developed in attempts to make such filter systems reusable, the most common being thermal and catalytic oxidation of filter-trapped particulates. Unfortunately, the space, cost, and energy consumption required by such regeneration methods are substantial. Furthermore, in-situ filter regeneration techniques, where the filters rely on the hot exhaust gases themselves to raise the temperature of the filter high enough to oxidize the trapped particles, do not work with the light duty-cycles typically associated with underground engines. Consequently, the high temperature filters used on such light duty cycle engines must be removed and regenerated at some off-site location.
Some systems have been developed in which the relatively expensive high temperature filters are replaced with cheaper, preferably disposable, low temperature filters. Water scrubbers are used to cool the exhaust gases before they pass through the low temperature filters. However, the moist exhaust gases exiting the water scrubbers tend to foul and clog the low temperature filters quite rapidly, and, as mentioned above, water scrubbers have their own problems and maintenance costs.
Finally, Liu et al. teach an electrostatic particle collector to remove the particulate matter from the exhaust gases. However, the high voltages required by this kind of system introduce yet another explosion hazard when used in underground environments or other environments having inflammable atmospheres.
Consequently, there still remains a substantial need for an improved diesel engine emission reduction system that is suitable for use in underground mines or in other environments that have explosive or poorly ventilated atmospheres, or in environments where it is essential that the quantities of solid particulates in the diesel exhaust be kept to a minimum. Such a system must meet the rigorous requirements for spark and flame suppression and for maximum surface and exhaust temperature, while at the same time providing an economical and low maintenance method of removing the solid particulate matter, and preferably some of the carbon monoxide and other pollutants, from the exhaust gases. Ideally, such a system would include a dry heat exchanger to allow the use of inexpensive, disposable low temperature filters and to eliminate the water supply requirements of a water scrubber, but without the need to frequently disassemble and clean the soot accumulations from the heat exchanger. Until the present invention, no such system existed.